Please post your program that displays in 24 hour format.
At its crudest you could simply test whether the hour is greater than 12 and if so subtract 12 before displaying it. If you want AM/PM displayed, do the same test and display the appropriate text.
Looks to me like you need to modify this line in virtualrobotixRTC.cpp from 0 to 1:
rtc.h24.hour_12_24 = 0; // 0 for 24 hour format
According to notepad++ it's line 653.
Or I suppose in code you could go:
myRTC.h24.hour_12_24 = 1;